As I said, there are other available spells. Not every mook needs Magic Missile to be their one-trick. Burning Hands, Snowball, and other 1st level spells with level increases to damage are all quite potent.
In regards to Potent Magic, its advantage is that it is easily worth a feat, and is a free action. Early game you have more points in your Pool than you have 1st level spells to cast daily. Its not until 4th level that Arcanists get more spell-slots than they have starting points in their Arcane Reservoir. As such, every single spell they cast during the day can benefit from Potent Magic. By level 3, with both Spell Foci feats, they can be rocking +4 to their DCs or with a Focus into Specialization they can have +4 to their caster level. (Scorching Ray is nice because it fires 2 beams with this strat at level 4). This is very strong early game, ludicrously so if you build towards Save or Suck or Level Dependent spells.
Remember, people legitimately spend a feat to get +1 to DC. Arcanists can just do it as a class feature, and get the feat on top of that.
In regards to the Mook not having the ability to grow stronger and this not making sense in the fluff, for one thing education is highly sedentary. Especially in the time periods that Pathfinder mirrors, and for goodness sake experimenting with Magic is a damn good way to get yourself exploded. Secondly, most of the population is NPC classes and making it to a PC class is a pretty solid achievement. Third, they absolutely can grow. Spell Specialization can be retrained every even level, and Spell Focus is a universally solid feat for a caster. You could run these guys as Tucker's Kobolds if you really wanted to, or more like Goblins really. The one that gets lucky enough not to die, thanks to being on the backline, kill a PC through luck, and then escape with a Vanish spell goes DING. Fourth, When you are made out of wet tissue paper having all of your power set to go off in the first two rounds gives you the best chance of surviving. Once they hit level 2, they can keep a spell in reserve to sneak/run away. Fifth, choosing to Alpha Strike makes perfect sense when deployed in groups. They either overwhelm what they encounter or die instantly.
The higher level Arcanists are more diverse in their casting ability, but still rely upon the very same idea of slamming people with their biggest spells first. Since Potent Magic is a free action both Quickened and regular spells can benefit from it, and by golly can that hurt.
Not really; if they're all part of the same organization or group and were explicitly trained in the same way then it makes sense. Also there is a degree of allowance for the GM in that they're not going to create unique stat-blocks for nameless mooks.A dark organisation training Arcanist from a young age to all go into the same build seems far fetched with the end game being, slightly more impactful mooks.
Not to mention that they're all supposed to be very intelligent, yet are fully willing to all follow exactly the same build which suggests doing exactly the same thing in their academic study as everyone else they know. To an inquisitive mind that would surely be boring and I don't see why they'd willingly go along with it.
Honestly I'd need some pretty good fluff to justify these overly complex mooks. But hey that's just me.
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They were trained in North Korea Bachuan.